1. Technical Field of the Invention
This invention relates generally to the field of energy harvesting, and more particularly, but not entirely, to piezoelectric energy harvesters.
2. Description of Related Art
Energy harvesting, also known as power harvesting or energy scavenging, is the process by which energy is derived from external sources, collected and stored. Energy harvesting devices can include devices that harvest energy from solar, thermal, wind, wave and other kinetic sources. Typically, thin film energy harvesters provide the power for wearable devices or stretchable electronics. However, despite the low-energy production of these energy harvesters, they have a small form factor, and they have the advantage of tapping energy from freely available sources.
Energy harvesting devices can be utilized to power low-power electronics. For example, harvested energy can be utilized to power sensors at remote locations where traditional power sources are unavailable or are too expensive. The energy generated by an energy harvesting device can be stored in a capacitor or a battery.
Piezoelectric materials have generated considerable interest as energy harvesters because of their unique properties. In particular, piezoelectric materials generate a small voltage whenever they are mechanically deformed. Generally speaking, piezoelectric materials fall into three different categories: piezoelectric ceramics, piezoelectric polymers and piezoelectric co-polymers.
One widely studied piezoelectric polymer is polyvinylidene fluoride (“PVDF”). PVDF is a highly non-reactive and pure thermoplastic fluoropolymer. One advantage to the use of PVDF is that it is low cost compared to other fluoropolymers. PVDF is not naturally a piezoelectric material. In order to use PVDF as a piezoelectric, it must be properly prepared. Preparation of PVDF as a piezoelectric includes producing thin films of PVDF that are then turned from an inert polymer to a poled piezoelectric film by increasing polarization density. Typically, polarization is increased by stretching and heating the PVDF film.
Attempts have been made to utilize PVDF, in its piezoelectric form, to harvest energy. One previous attempt included using PVDF to harvest wind energy. In particular, gold electrodes were attached to either side of a small, thin film sheet of PVDF. Leads connected the gold electrodes to a simple charging circuit. The PVDF sample was then placed in a vortex created in a wind tunnel which caused the PVDF sample to oscillate. While the PVDF sample in this attempt initially produced good results, after a short period of time, as short as one or two hours, the gold electrodes began to crack from stress caused by the oscillation of the PVDF sample in the wind current.
In addition, the thin film of the PVDF sample itself showed signs of stress fatigue from the oscillations. This attempt was therefore deemed unsuitable for a real-world application because of the short life of the PVDF sample and the gold electrodes in the lab. Attempts were made to use commercially available non-metal conductive epoxies as electrodes. However, these thicker electrode materials unduly dampened the mechanical strains necessary to generate energy.
It would therefore be an improvement over the previously available devices to provide piezoelectric materials and electrodes that are more compliant and less susceptible to fatigue in real-world applications. It would further be an improvement over the previously available devices to provide electrodes for use with piezoelectric devices that are compliant and that do not overly interfere with the generation of energy. It would further be an improvement over the previously available devices to provide electrodes for use with piezoelectric devices that have an improved mechanical and interfacial bonding to the piezoelectric material and that are flexible enough to allow large strains in the piezoelectric material to occur and that remain electrically conductive upon application of large strains and repeated cyclic loadings.
The features and advantages of the present disclosure will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by the practice of the present disclosure without undue experimentation. The features and advantages of the present disclosure may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.